Frequency with which charged particles spin around a magnetic field line.
The force F exerted on a particle of charge q travelling at a velocity v in a magnetic field B is given by F=q(v curl B) Since this force is perpendicular to both the magnetic field and the particle velocity, it results in a spiral motion around the magnetic field at the cyclotron frequency ωcωc=qB/m where m is the particle mass. (See Larmor radius writeup for more details).Thus, a strong magnetic field produces fast spinning ions and electrons, though ions will spin less frequently due to their greater mass.
Oscillations at the cyclotron frequency occur, naturally enough, in cyclotrons, and in plasmas. | 
